1. Field of the Invention
The present invention relates to an image monitoring apparatus such as a radar apparatus, a scanning sonar apparatus and a course plotting apparatus, for example.
2. Description of the Prior Art
Conventionally known technology used in a radar apparatus, for example, includes a zoom function which allows an operator to enlarge a desired portion of a whole radar coverage for observing smaller details within a particular area of interest. A commonly used technique in this kind of zoom function is to place a cross-hair cursor (a pair of intersecting lines for specifying a particular location on a screen) at the center of an area to be zoomed and operate a key to execute the zoom function, whereby an enlarged image of the area around the cursor position is presented on the screen at a varied range scale.
A conventional scanning sonar transmits and receives ultrasonic signals at a set tilt angle and provides a two-dimensional presentation of underwater objects detected within a whole sounding area around the ship on a display screen.
A course plotting device of a conventional type successively determines positions of a ship and displays its past track overlaid with an electronically generated chart of a relevant area on a screen. The course plotting device usually has the capability to display the ship's plotted track and chart on the entire screen in three presentation modes: i.e., north-up mode in which the geographical north is maintained in the screen top direction, head-up mode in which the ship's heading is directed in the screen top direction, and course-up mode in which the ship's intended course is stabilized in the screen top direction.
The zoom function of the aforementioned conventional radar provides an enlarged view of target blips existing in an operator-selected area of interest (zooming area) within the whole radar coverage. This function enables the operator to observe that particular area with special attention focused thereon. When the zoom function is executed on the conventional radar, however, the on-screen image is entirely overwritten. The zoom function of the prior art has, therefore, been associated with a problem that it is difficult to recognize a positional relationship between the whole radar coverage and the zooming area.
A previous approach directed to the solution of the above problem is, as disclosed in Japanese Unexamined Patent Publication No. 63-241480 (1988), Japanese Unexamined Utility Model Publication No. 2-75588 (1990) and Japanese Unexamined Patent Publication No. 5-273348 (1993), to present a plurality of images in different range scales in respective display areas (or "windows") provided on a single screen. According to such a prior art windowing technique, the images displayed in the individual windows on the screen have a common orientation in azimuth. In other words, it has been impossible to present radar images with different reference directions in the individual windows to meet specific requirements. Although the prior art technique makes it possible to present an enlarged image of a particular portion specified within the whole radar coverage in a separate zooming window for focusing on targets within that portion, the targets shown in the enlarged image would easily go out of the zooming window if an observer's own ship travels a certain distance or alters its heading.
It has not been possible for either the conventional scanning sonar or course plotting device to show a plurality of windows on a single screen and present part of an image shown on one window on another window.